Sedevacantists like to take the Archbishop out of context and make him appear a closet Sedevacantist.
Apart from the Father Cekada's video, where he's clearly trolling R&R by entitling the video, "Marcel Lefebvre: Sedevacantist", I've never known a sedevacantist who claimed that +Lefebvre was a "closet Sedevacantist". Most SVs recognize the objective truth, which some of the obsessive/dogmatic R&R like Plenus deny.
1) Apart from a period in the early 1980s, +Lefebvre was not particularly hostile to SVism.
2) There were times, 1976, 1986, 1988 where he flirted with it and appeared to come close to embracing it.
3) Unlike many of his later R&R followers, +Lefebvre did not deny that the Papacy is protected by the Holy Ghost and prevented from destroy the Church to the degree that the V2 popes did, thereby upholding the MAJOR premise of sedevacantism, but simple felt that the MINOR (the explanation for how this happened) was uncertain, deferring it to the Church's future judgment.
That is the objective reality that some / many of his followers refuse to admit, because they're intellectually dishonest.
See, here's the thing. SVs can be objective because they don't uphold +Lefebvre as some kind of substitute rule of faith for the Magisterium. "+Lefebvre did not embrace sedevacantism 100%. So what? He was wrong." Meanwhile, many R&R have replaced the actual Catholic / papal Magisterium with the teaching of +Lefebvre (which changed at different times, so that their "Magisterium" is changeable, like that of the Modernist, ironically). This is to fill the vacuum of the Magisterium that they discard.
Father Cekada astutely points out that at different points in SSPX history, the so-called "hard-liners" were on the outs, and at other times the "soft-liners", but at all times the +Lefebvre-liners, the sycophants who didn't think but just believed as dogmatic truth whatever the Archbishop said on any given morning, thrived and were promoted into positions of authority, even if it means holding the opposite opinion from one day to the next.
Back to the Archbishop, he was not "dishonest" or "two-faced" or a "flip-flopper". Archbishop Lefebvre was truly conflicted by the contradiction posed by the Conciliar Church, almost tormented by it. This is a conflict that many modern R&R, like Plenus here, don't have, because they've discarded the one pole of that conflict, namely, Archbishop Lefebvre's conviction that the Papacy and the Church are protected and guided by the Holy Ghost and prevented from destroying the Church. He couldn't resolve that principle with the reality of a Montini or Wojtyla. So that's why he termed it a "mystery" and left it there, unable to definitively resolve the conflict in his mind. But modern R&R have conveniently disposed of the one side or the one pole in that conflict, namely, the protection of the papacy and the Magisterium by the Holy Ghost. That's why, sadly, many modern R&R have devolved into a form of Old Catholicism.